Fluoroalkylethnyl- and difluoroalkylethynylbenzenes, and their use in liquid-crystal mixtures

ABSTRACT

PCT No. PCT/EP95/01766 Sec. 371 Date Nov. 18, 1996 Sec. 102(e) Date Nov. 18, 1996 PCT Filed May 10, 1995 PCT Pub. No. WO95/32261 PCT Pub. Date Nov. 30, 1995Disclosed are derivatives of fluoroalkylethynyl- and difluoroalkylethynylbenzenes of the formula (I) herein. These compounds exhibit useful properties as liquid crystal compounds and are useful in nematic liquid crystal mixtures.

The unusual combination of anisotropic and fluid behavior of liquidcrystals has resulted in their use in electro-optical switching anddisplay devices, where their electrical, magnetic, elastic and/orthermal properties can be utilized for changes in alignment. Opticaleffects can be achieved, for example, with the aid of birefringence, ofthe inclusion of dichroic dye molecules (guest host mode), or of lightscattering.

The requirements of practice are constantly rising, not least also owingto the constantly increasing number of types of light valve (TN, STN,DSTN, TFT, ECB, DECB, DS, GH, PDLC, NCAP, SSFLC, DHF, SBF, etc.).Besides thermodynamic and electro-optical parameters, such as phasesequence and phase temperature range, refractive index, birefringenceand dielectric anisotropy, response time, threshold voltage, steepnessof the electro-optical characteristic line, elastic constants,electrical resistance, multiplexability or pitch and/or polarization inchiral phases, the stability of the liquid crystals to moisture, gases,temperature and electromagnetic radiation and to the materials withwhich they are in contact during and after the production process (forexample alignment layers) is of considerable importance. Toxicologicaland ecological acceptability and price are constantly increasing inimportance.

A broad review of the field of liquid crystals is offered, for example,by the literature below and the references cited therein:

H. Kelker, H. Hatz, Handbook of Liquid Crystals, Verlag Chemie,Weinheim, 1980;

W. E. De Jeu, Physical Properties of Liquid Crystal Materials, Gordonand Breach, Philadelphia, 1980;

H. Kresse, Dielectric Behaviour of Liquid Crystals, Fortschritte derPhysik, Berlin 30, 10, 1982, 507-582;

B. Bahadur, Liquid Crystals: Applications and Uses, World Scientific,Singapore, 1990;

Landolt-Bornstein, New Series, Group IV, Volume 7 Liquid Crystals,1992-1993 and

J. W. Goodby et al., Ferroelectric Liquid Crystals: Principles,Properties and Applications, Gordon Breach, Philadelphia, 1991.

WO 90/13610, EP 0 480 217 and DE-A-40 27 458 describe derivatives oftrifluoromethylethynyl-, fluoroethynyl- and chloroethynylbenzene for usein liquid-crystal mixtures.

Since, however, individual compounds have hitherto been unable tosatisfy simultaneously the many different requirements of liquidcrystals, some of which are mentioned above, there is a constant demandfor novel improved liquid-crystal mixtures and thus for a multiplicityof mesogenic and non-mesogenic compounds of different structure whichenable the mixtures to be adapted to a very wide variety ofapplications. This applies both to the areas in which nematic LC phasesare used in light valves and to those having smectic phases.

Surprisingly, it has now been found that derivatives offluoroalkylethynyl- and difluoroalkylethynylbenzene are particularlysuitable for use in nematic liquid-crystal mixtures.

The invention thus relates to novel fluoroalkylethynyl- anddifluoroalkylethynylbenzenes of the general formula (I) ##STR1## inwhich the symbols and indices have the following meanings: R¹ is H, astraight-chain or branched (with or without an asymmetric carbon atom)alkyl group having 1 to 15 carbon atoms, in which, in addition, one ortwo non-adjacent CH₂ groups may be replaced by --O--, --S--, --CO--,--CO--O--, --O--CO--, --CO--S--, --S--CO--, --O--CO--O--, --CH═CH--,--C.tbd.C--, cyclopropane-1,2-diyl or --Si(CH₃)₂ --, and in which, inaddition, one or more H atoms in the alkyl radical may be substituted byF, Cl or CN;

A¹, A² and A³ are identical or different and are 1,4-phenylene,pyrazine-2,5-diyl, pyridine-2,5-diyl, pyrimidine-2,5-diyl, in which oneor two H atoms may be replaced by F, trans-1,4-cyclohexylene,1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl, naphthalene-2,6-diyl,bicyclo 2.2.2!octane-1,4-diyl or 1,3-dioxoborinane-2,5-diyl;

M¹, M² and M³ are identical or different and are --CH₂ CH₂ --,--CH═CH--, --C.tbd.C--, --CH₂ CH₂ CH₂ CH₂ --, --CH₂ CH₂ CH₂ --O--,--O--CH₂ CH₂ CH₂ --, --CH₂ CH₂ --CO--O--, --O--CO--CH₂ CH₂ --, --CH₂--O--, --O--CH₂ --, --CO--O-- or --O--CO--;

k, l, m, n, o and p are zero or one, with the proviso that the sum k+m+ois greater than zero;

X¹, X², X³ and X⁴ are, independently of one another, H, F or Cl;

Y is a fluorinated alkyl radical having 1 to 3 carbon atoms.

The novel compounds of the general formula (I) are chemically andphotochemically stable. They have low melting points and generally havebroad liquid-crystalline phases, in particular broad nematic phases.

In particular, the novel compounds have very high anisotropy of therefractive index and of the dielectric constant, which makes themparticularly suitable for use in TFT-TN and STN mixtures., In apreferred embodiment of the invention, the symbols and indices in theformula (I) have the following meanings:

R¹ is a straight-chain alkyl having 1 to 15 carbon atoms;

A¹, A² and A³ are identical or different and are 1,4-phenylene, in whichone or two H atoms may be replaced by F, or trans-1,4-cyclohexylene;

M¹, M² and M³ are identical or different and are --CH₂ CH₂ --,--C.tbd.C--, --CH₂ CH₂ CH₂ CH₂ --, --CH₂ CH₂ CH₂ --O--, --O--CH₂ CH₂ CH₂--, --CH₂ CH₂ --CO--O--, --O--CO--CH₂ CH₂ --, --CH₂ --O--, --O--CH₂ --,--CO--O-- or --O--CO--;

k, l, m, n, o and p are zero or one, with the proviso that the sum k+m+ois greater than zero;

X¹, X² ₁ X³ and X⁴ are, independently of one another, H or F; and

Y is preferably --CH₂ F or --CHF₂.

Particularly preferred compounds of the formula (I) are thefluoroalkylethynyl- and difluoroalkylethynylbenzenes of the formulae(I1) to (I44) listed below: ##STR2## where R¹ is methyl, ethyl, propyl,butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl, and L¹ and L² areeach, independently of one another, H or F.

The preparation of the novel compounds can be effected by methods knownper se from the literature (see, for example, Houben-Weyl, Methoden derOrganischen Chemie, Georg Thieme-Verlag, Stuttgart), and is illustratedby way of example in Scheme 1. ##STR3##

Cross-coupling of aromatic bromides or iodides of type (II) withtrimethylsilylacetylene with catalysis by palladium compounds, such asbis(triphenylphosphine)palladium dichloride, gives thetrimethylsilylethynylbenzenes (III).

Desilylation of species (III) by means of a suitable base, for examplepotassium hydroxide, gives the ethynylbenzenes (IV), which, afterdeprotonation, for example using n-butyllithium, and reaction with theelectrophiles dimethylformamide or formaldehyde, gives theformylethynylbenzenes (V) and the hydroxymethylethynylbenzenes (VI) (seein this respect: S. Takahashi, Y. Kuroyama, K. Sonogashira, N. Hagihara,Synthesis 1980, 627).

Reaction of the compounds of type (V) and (VI) with diethylaminosulfurtrifluoride (DAST) gives the novel fluoromethylethynyl- anddifluoromethylethynylbenzenes (I) (see in this respect: M. Hudlicky inOrganic Reactions, Vol. 35, p. 513).

The above-described process is likewise a subject-matter of theinvention.

The radical ##STR4## is synthesized by methods known per se andcustomary to the person skilled in the art.

This preparation is carried out under reaction conditions which areknown and suitable for said reactions. One may also make use here ofvariants which are known per se but are not mentioned here in greaterdetail.

Reference may be made, for example, to DE-A 23 44 732, 24 50 088, 24 29093, 25 02 94, 26 36 684, 27 01 591 and 27 52 975 for compoundscontaining 1,4-cyclohexylene and 1,4-phenylene groups; DE-A 26 41 724for compounds containing pyrimidine-2,5-diyl groups; DE-A 40 26 223 andEP-A 03 91 203 for compounds containing pyridine-2,5-diyl groups; DE-A32 31 462 for compounds containing pyridazine-3,6-diyl groups, EP-A 309514 for compounds containing 1,3,4-thiadiazole-2,5-diyl groups, WO92/16500 for naphthalene-2,6-diyl groups; DE 3710-890 for bicyclo2.2.2!octane-1,4-diyl groups; K. Seto, H. Matsubara, S. Takahashi, T.Takara, M. Murakami, S. Miyake, T. Masumi, T. Ando, A. Fukami, Journalof the Chemical Society, Chemical Communications, 1988, 56, fordioxoborinane-2,5-diyl groups.

The preparation of disubstituted pyridines, disubstituted pyrazines anddisubstituted pyrimidines is also given, for example, in thecorresponding volumes of the series "The Chemistry of HeterocyclicCompounds" by A. Weissberger and E. C. Taylor (editors).

Dioxane derivatives are expediently prepared by reaction of anappropriate aldehyde (or a reactive derivative thereof) with anappropriate 1,3-diol (or a reactive derivative thereof), preferably inthe presence of an inert solvent, such as benzene or toluene, and/or inthe presence of a catalyst, for example a strong acid, such as sulfuricacid or benzene- or p-toluenesulfonic acid, at temperatures betweenabout 20° C. and about 150° C., preferably between 80° C. and 120° C.Suitable reactive derivatives of the starting materials are primarilyacetals.

Some of said aldehydes and 1,3-diols and reactive derivatives thereofare known, and some can be prepared without difficulties from compoundsknown from the literature by standard methods of organic chemistry. Forexample, the aldehydes are obtainable by oxidation of correspondingalcohols or by reduction of nitriles or corresponding carboxylic acidsor derivatives thereof, and the diols by reduction of correspondingdiesters.

Compounds in which an aromatic ring carries at least one F atom assubstituent can also be obtained from the corresponding diazonium saltsby replacement of the diazonium group by a fluorine atom, for example bythe Baltz and Schiemann methods.

Regarding linking of the ring systems to one another, reference is madeto: N. Miyaura, T. Yanagai and A. Suzuki, Synthetic Communications 1981,11, 513-519; DE-C-39 30 663, M. J. Sharp, W. Cheng, V. SnieckusTetrahedron Letters 1987, 28, 5093; G. W. Gray, J. Chem. Soc. PerkinTrans II 1989, 2041 and Mol. Cryst. Liq. Cryst. 1989, 172, 165; 1991,204, 43 and 91; EP-A 0 449 015; WO 89/12039; WO 89/03821 and EP-A 0 3544 34 for the direct linking of aromatics and heteroaromatics; DE-A 32 01721 for compounds containing --CH₂ CH₂ -- bridges, and Koji Seto et al.,Liquid Crystals 1990, 8, 861-870, for compounds containing --C.tbd.C--bridges.

Esters of the formula (I) can also be obtained by esterification ofcorresponding carboxylic acids (or reactive derivatives thereof) usingalcohols or phenols (or reactive derivatives thereof) or by the DCCmethod (DCC=dicyclohexylcarbodiimide).

The corresponding carboxylic acids and alcohols or phenols are known andcan be prepared analogously to known processes.

Particularly suitable reactive derivatives of said carboxylic acids arethe acid halides, especially the chlorides and bromides, and furthermorethe anhydrides, for example also mixed anhydrides, azides or esters, inparticular alkyl esters having 1 to 4 carbon atoms in the alkyl group.

Suitable reactive derivatives of said alcohols and phenols are, inparticular, the corresponding metal alkoxides and phenoxides, preferablyof an alkali metal, such as sodium or potassium.

The esterification is advantageously carried out in the presence of aninert solvent. Particularly suitable solvents are ethers, such asdiethyl ether, di-n-butyl ether, THF, dioxane or anisole, ketones, suchas acetone, butanone or cyclohexanone, amides, such as DMF orhexamethylphosphoric triamide, hydrocarbons, such as benzene, toluene orxylene, halogenated hydrocarbons, such as tetrachloromethane,dichloromethane or tetrachloroethylene, and sulfoxides, such as dimethylsulfoxide or sulfolane.

Ethers of the formula (I) are obtainable by etherification ofcorresponding hydroxyl compounds, preferably corresponding phenols,where the hydroxyl compound is expediently first converted into acorresponding metal derivative, for example into the correspondingalkali metal alkoxide or alkali metal phenoxide by treatment with NaH,NaNH₂, NaOH, KOH, Na₂ CO₃ or K₂ CO₃. This can then be reacted with thecorresponding alkyl halide, sulfonate or dialkyl sulfate, expediently inan inert solvent, such as acetone, 1,2-dimethoxyethane, DMF or dimethylsulfoxide, or alternatively with an excess of aqueous oraqueous-alcoholic NaOH or KOH at temperatures between about 20° C. and100° C.

Regarding the synthesis of specific radicals R¹, reference mayadditionally be made, for example, to EP-A 0 355 008 for compoundshaving silicon-containing side chains, and EP-A 0 292 954 and EP-A 0 398155 for compounds with cyclopropyl groups in the side chain.

The provision of compounds of the formula (I) very generallyconsiderably broadens the range of liquid-crystalline substances whichare suitable, from various applicational standpoints, for thepreparation of liquid-crystalline mixtures.

The compounds of the formula (I) have a broad range of applications.Depending on the choice of substituents, these compounds can be used asbase materials from which liquid-crystalline phases are predominantlycomposed; or, however, compounds of the formula (I) can be added toliquid-crystalline base materials from other classes of compound inorder, for example, to modify the dielectric and/or optical anistropy ofa dielectric of this type and/or to optimize its threshold voltageand/or its viscosity.

In the pure state, the compounds of the formula (I) are colorless andform liquid-crystalline mesophases in a temperature range which isfavorably located for electro-optical use. They are stable chemically,thermally and with respect to light.

Compounds of the formula (I) can be used for the preparation of nematicor alternatively chirally nematic liquid-crystal mixtures, which aresuitable for use in electro-optical or fully optical elements, forexample display elements, switching elements, light modulators, elementsfor image processing, signal processing or generally in the area ofnonlinear optics.

The invention therefore also relates to the use of compounds of theformula (I) in liquid-crystal mixtures, in particular nematic andchirally nematic liquid-crystal mixtures.

The invention also relates to liquid-crystal mixtures comprising one ormore compounds of the formula (I).

The novel liquid-crystal mixtures generally consist of from 2 to 20,preferably from 2 to 15, components, including at least one, preferablyfrom 1 to 5, particularly preferably from 1 to 3, compounds of theformula (I). The novel LC mixtures can be, for example, nematic orchirally nematic. The novel liquid-crystal mixtures generally comprisefrom 0.1 to 70 mol %, preferably from 0.5 to 50 mol %, in particularfrom 1 to 25 mol %, of the novel fluoromethylethynyl- anddifluoromethylethynylbenzene derivatives of the formula (I). Furtherconstituents of the novel mixtures are preferably selected from knowncompounds having nematic or cholesteric phases, including, for example,biphenyls, terphenyls, phenylcyclohexanes, bicyclohexanes,cyclohexylbiphenyls, and mono-, di- and trifluorophenyls. In general,commercially available liquid-crystal mixtures, even before addition ofthe novel compound(s), are mixtures of various components, of which atleast one is mesogenic.

Suitable further constituents of novel nematic or chirally nematicliquid-crystal mixtures are, for example,

4-fluorobenzenes, as described, for example, in EP-A 494 368, WO 92/06148, EP-A 460 436, DE-A 41 11 766, DE-A 41 12 024, DE-A 41 12 001, DE-A41 00 288, DE-A 41 01 468, EP-A 423 520, DE-A 392 3064, EP-A 406 468,EP-A 393 577, EP-A 393 490,

3,4-difluorobenzenes, as described, for example, in DE-A 41 08 448, EP-A507 094 and EP-A 502 407,

3,4,5-trifluorobenzenes, as described, for example, in DE-A 41 08 448and EP-A 387 032,

4-benzotrifluorides, as described, for example, in DE-A 41 08 448, and

phenylcyclohexanes, as described, for example, in DE-A 41 08 448.

The novel liquid-crystal mixtures are prepared in a manner which isconventional per se. In general, the components are dissolved in oneanother, preferably at elevated temperature.

Liquid-crystal mixtures which comprise compounds of the general formula(I) are particularly suitable for use in electro-optical switching anddisplay devices (displays). These displays are usually constructed insuch a way that a liquid-crystal layer is surrounded on both sides bylayers which are usually, in this sequence starting from the LC layer,at least one alignment layer, electrodes and a limiting plate (forexample made of glass). In addition, they contain spacers, adhesiveframes, polarizers and, for color displays, thin colored filter layers.Other possible components are anti-reflection, passivation, equalizationand barrier layers and electrically nonlinear elements, such asthin-film transistors (TFTs) and metal-insulator-metal (MIM) elements.The structure of liquid-crystal displays has already been described indetail in relevant monographs (for example E. Kaneko, "Liquid Crystal TVDisplays: Principles and Applications of Liquid Crystal Displays", KTKScientific Publishers, 1987, pages 12-30 and 63-172).

The novel liquid-crystal mixtures are particularly suitably for use inSTN (super twisted nematics) and TFT-TN (thin film transistor-twistednematics) displays. Such display types are known and described, forexample, in B. Bahadur (Ed.), Liquid Crystals, Applications and Uses(Vol. 1), World Scientific, Singapore, 1990, in Chapters 10, p. 231-274(STN), and 15, p. 397-436 (TFT-TN).

The invention is described in greater detail by the examples without thedesire for it to be limited thereby.

EXAMPLES

Various measurement methods are used for physical characterization ofthe novel compounds.

The phase transition temperatures are determined with the aid of apolarizing microscope from the changes in structure on heating. Bycontrast, the melting point is determined using a DSC instrument. Thephase transition temperatures between the phases

Isotropic (I)

Nematic (N or N*)

Smectic C (S_(C) or S_(C) *)

Smectic A (S_(A) or S_(A) *)

Crystalline (X)

Glass transition (Tg)

are given in °C., and the values are between the phase designations inthe phase sequence.

If the values for heating and cooling are different, the latter areplaced in parentheses, or the phase sequence is given with rising orfalling temperature.

Electro-optical investigations are carried out by methods known from theliterature (for example B. Bahadur: Liquid Crystals Application andUses, Vol. 1, World Scientific, Singapore, 1990).

For nematic liquid crystals (pure or mixtures), the values for theoptical and dielectric anisotropy and for the electro-opticalcharacteristic line are recorded at a temperature of 25° C.

Liquid crystals which do not have a nematic phase at 25° C. are mixed tothe extent of 10% by weight with ZLI-1565 and/or to the extent of 20% byweight with ZLI-4792 (commercial nematic liquid-crystal mixtures from E.Merck, Darmstadt), and the values are extrapolated from the results forthe mixture.

Electro-optical characteristic lines are determined from thetransmission of a measurement cell. To this end, the cell is positionedbetween crossed polarizers in front of a light source. A light detectorwhose sensitivity is optimized to the visible region of the light bymeans of filters is positioned behind the cell. The change intransmission is recorded analogously to the stepwise increase in thevoltage applied to the cell. Parameters such as threshold voltage andsteepness are determined therefrom.

The optical anistropy is determined using an Abbe refractometer (Zeiss).In order to align the liquid crystal, an alignment layer obtained from a1% by weight lecithin/methanol solution is applied to the prism.

In order to determine the dielectric anisotropy, a measurement cellhaving homeotropic and planar alignment is in each case constructed andtheir capacitances and dissipation factors determined using amultifrequency LCR meter (Hewlett Packard 4274 A). The dielectricconstants are calculated as described in the literature (W. Maier, G.Meier, Z. Naturforsch, 1961, 16a, 262 and W. H. de Jeu, F. Leenhonts, J.Physique 1978, 39, 869). The electrical parameter HR (holding ratio) isdetermined as described in the literature (M. Schadt, Linear andnonlinear liquid crystal materials, Liquid Crystals 1993, 14, 73-104).

In order to determine the response speed (τ) and contrast (C), themeasurement cell is clamped on the rotating stage of a polarizingmicroscope between crossed analyzer and polarizer. For determination ofthe contrast, the measurement cell is positioned by rotation so that aphotodiode displays minimal light transmission (dark state). Themicroscope illumination is adjusted so that the photodiode indicates thesame light intensity for all cells. After a switching operation, thelight intensity changes (bright state), and the contrast is calculatedfrom the ratio between the light intensities in these states.

Example 1 1-(Fluoromethylethynyl)-4-(trans-4-pentylcyclohexyl)benzene:

10.00 g (32.36 mmol) of 1-bromo-4-(trans-4-pentylcyclohexyl)benzene,3.89 g (35.60 mmol) of trimethylsilylacetylene, 0.45 g (0.65 mmol) ofbis(triphenylphosphine)palladium(II) chloride and 0.062 g (0.32 mmol) ofcopper(I) iodide are heated for 6 hours at the reflux temperature in 100ml of diisopropylamine. The solvent is subsequently stripped off, andthe soluble constituents of the residue are chromatographed on silicagel using n-hexane, giving 7.32 g of1-(trimethylsilylethynyl)-4-(trans-4-pentylcyclohexyl)benzene, ##STR5##which is stirred for 1 hour at room temperature in 220 ml of methanolwith 1.38 g (24.70 mmol) of potassium hydroxide. Work-up by evaporationof the solvent, partitioning between ether and water, drying andevaporation of the organic phase gives 4.95 g of1-ethynyl-4-(trans-4-pentylcyclohexyl)benzene. ##STR6##

6.75 ml (10.80 mmol) of 1.6 molar n-butyllithium solution in n-hexaneare added dropwise at 0° C. to 2.48 g of1-ethynyl-4-(trans-4-pentylcyclohexyl)benzene in 25 ml oftetrahydrofuran. After 1 hour, this solution is saturated withformaldehyde, acidified by means of aqueous hydrochloric acid andpartitioned between ether and water, and the organic phase is dried andevaporated. Chromatography on silica gel using n-hexane/dichloromethane(1:2) gives 1.80 g of1-(hydroxymethylethynyl)-4-(trans-4-pentylcyclohexyl)benzene, ##STR7##which is stirred for 2 hours at room temperature with 1.05 g (6.49 mmol)of diethylaminosulfur trifluoride in 10 ml of dichloromethane. Afterpartitioning between water and dichloromethane, the organic phase isdried over sodium sulfate, evaporated and chromatographed on silica gelusing n-heptane, giving 0.94 g of1-(fluoromethylethynyl)-4-(trans-4-pentylcyclohexyl)benzene. ##STR8##

Examples 2 to 18 shown in Table 1 are prepared analogously to Example 1:

Table 1: Examples 2 to 30

    __________________________________________________________________________    Example No.                                                                           ##STR9##                                                              __________________________________________________________________________     2                                                                                    ##STR10##                                                              3                                                                                    ##STR11##                                                              4                                                                                    ##STR12##                                                              5                                                                                    ##STR13##                                                              6                                                                                    ##STR14##                                                              7                                                                                    ##STR15##                                                              8                                                                                    ##STR16##                                                              9                                                                                    ##STR17##                                                             10                                                                                    ##STR18##                                                             11                                                                                    ##STR19##                                                             12                                                                                    ##STR20##                                                             13                                                                                    ##STR21##                                                             14                                                                                    ##STR22##                                                             15                                                                                    ##STR23##                                                             16                                                                                    ##STR24##                                                             17                                                                                    ##STR25##                                                             18                                                                                    ##STR26##                                                             19                                                                                    ##STR27##                                                             20                                                                                    ##STR28##                                                             21                                                                                    ##STR29##                                                             22                                                                                    ##STR30##                                                             23                                                                                    ##STR31##                                                             24                                                                                    ##STR32##                                                             25                                                                                    ##STR33##                                                             26                                                                                    ##STR34##                                                             27                                                                                    ##STR35##                                                             28                                                                                    ##STR36##                                                             29                                                                                    ##STR37##                                                             30                                                                                    ##STR38##                                                             __________________________________________________________________________

Example 31 1-(Difluoromethylethynyl)-4-(trans-4-pentylcyclohexyl)benzene

6.75 ml (0.80 mmol) of 1.6 molar n-butyllithium solution in n-hexane areadded dropwise at 0° C. to 2.48 g (9.80 mmol) of1-ethynyl-4-(trans-4-pentylcyclohexyl)benzene in 25 ml oftetrahydrofuran. After 1 hour, this solution is added dropwise to 0.83ml of dimethylformamide in 20 ml of tetrahydrofuran, the mixture isstirred at room temperature for 1 hour, acidifed by means of aqueoushydrochloric acid and partitioned between ether and water, and theorganic phase is dried and evaporated. Chromatography on silica gelusing n-hexane/dichloromethane (1:1) gives 1.75 g of1-(formylethynyl)-4-(trans-4-pentylcyclohexyl)benzene, ##STR39## whichis stirred for 2 hours at room temperature with 1.05 g (6.49 mmol) ofdiethylaminosulfur trifluoride in 10 ml of dichloromethane. Afterpartitioning between water and dichloromethane, the organic phase isdried over sodium sulfate, evaporated and chromatographed on silica gelusing n-heptane, giving 0.89 g of product ##STR40## having the phasesequence

    X 37 (26) N (34) I.

Examples 32 to 59 shown in Table 2 are prepared analogously to Example31:

                                      TABLE 2                                     __________________________________________________________________________    Examples 32 to 59                                                             Example No.                                                                           ##STR41##                                                             __________________________________________________________________________    32                                                                                    ##STR42##                                                             33                                                                                    ##STR43##                                                             34                                                                                    ##STR44##                                                             35                                                                                    ##STR45##                                                             36                                                                                    ##STR46##                                                             37                                                                                    ##STR47##                                                             38                                                                                    ##STR48##                                                             39                                                                                    ##STR49##                                                             40                                                                                    ##STR50##                                                             41                                                                                    ##STR51##                                                             42                                                                                    ##STR52##                                                             43                                                                                    ##STR53##                                                                    Phase sequence: X 114 (55) S.sub.X 166 S.sub.A 204 N 217 I             44                                                                                    ##STR54##                                                             45                                                                                    ##STR55##                                                                    Phase sequence: X 87 (52) N 164 I                                      46                                                                                    ##STR56##                                                             47                                                                                    ##STR57##                                                             48                                                                                    ##STR58##                                                             49                                                                                    ##STR59##                                                             50                                                                                    ##STR60##                                                             51                                                                                    ##STR61##                                                             52                                                                                    ##STR62##                                                             53                                                                                    ##STR63##                                                             54                                                                                    ##STR64##                                                             55                                                                                    ##STR65##                                                             56                                                                                    ##STR66##                                                             57                                                                                    ##STR67##                                                             58                                                                                    ##STR68##                                                             59                                                                                    ##STR69##                                                             __________________________________________________________________________

USE EXAMPLES

The novel substances from Examples 31 and 45 are mixed to the extent of20% by weight with ZLI 4792 (commercially available TFT-TN mixture fromE. Merck, Darmstadt). Table 1 shows how the novel substances favorablyaffect the physical properties of the mixture.

The novel substances are distinguished by very high anisotropy of therefractive index and of the dielectric constants, which makes themparticularly suitable for STN and PDLC mixtures as well as for TFT-TNmixtures. In addition, the substances significantly lower the morehighly ordered, smectic phase transition, which is advantageous. Thesubstance from Ex. 45 has a very broad nematic phase with high clearingpoint and significantly extends the nematic phase range in mixturestoward high and low temperatures.

Table 2 shows that the pure substance from Ex. 31 has a low meltingpoint and in addition, surprisingly, a nematic phase, while theundesired smectic phases are not observed. Comparative Examples 1 to 3from WO 90/13610 exhibit, by contrast, significantly higher meltingpoints and also have undesired smectic phases.

                                      TABLE 1                                     __________________________________________________________________________                                                           Extrapolated                                          20% by weight in ZLI-4792                                                                             values for pure        Pure substance:                             25° C.                                                                            substance              Structure/phases               Phase sequence/°C.                                                                  Δn                                                                          Δε                                                                 U.sub.10                                                                          Δn                                                                          Δ.epsilon                                                               .                  __________________________________________________________________________     ##STR70##                     Tg - 75 (-77) X.sub.re - 37X - 6 S.sub.x                                      (-50) N 80 I 0.1031                                                                            5.2                                                                              1.97                                                                              0.148                                                                             7.8                X 37 (26) N 34 I                                                              Substance from Ex. 31                                                          ##STR71##                     Tg - 78 (-81) X - 44S.sub.x  (-56) N 105                                                   0.1235                                                                            6.96                                                                             1.85                                                                              0.222                                                                             13.7               X 87 (52) N 164 I                                                             Substance from Ex. 45                                                         ZLI-4792                       Tg - 82 (-84) X.sub.re - 63X                                                               0.0955                                                                            5.0                                                                              1.94                       (E. Merck, Darmstadt)          -31/-9S.sub.x (-44) N 94                       __________________________________________________________________________                                   I                                          

                                      TABLE 2                                     __________________________________________________________________________            Structure                 Phase sequence/°C.                   __________________________________________________________________________    Substance from Example 31                                                              ##STR72##                X 37 (26) N (34) I                          Comparative                                                                   Examples from                                                                 wo 90/12610                                                                   C1                                                                                     ##STR73##                X 63 S.sub.A (54) I                         C2                                                                                     ##STR74##                X 108 I                                     C3                                                                                     ##STR75##                X 77 I                                      __________________________________________________________________________

We claim:
 1. A fluoroalkylethynyl- or difluoroalkylethynyl- benzenecompound of the formula (I) ##STR76## wherein: R¹ is H, a straight-chainor branched (with or without an asymmetric carbon atom) alkyl grouphaving 1 to 15 carbon atoms, in which one or two non-adjacent CH₂ groupsare optionally replaced by --O--, --S--, --CO--, --CO--l--, --O--CO--,--CO--S--, --S--CO--, --O--CO--O--, --CH═--CH--, --C.tbd.C--,cyclopropane-1,2-diyl or --Si(CH₃)₂ --, and in which one or more H atomsin the alkyl group are optionally replaced by F, Cl or CN;A¹, A² and A³are identical or different and are 1,4-phenylene, pyrazine-2,5-diyl,pyridine-2,5-diyl or pyrimidine-2,5-diyl, in which one or two H atomsare optionally replaced by F, or trans-1,4-cyclohexylene,1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl, naphthalene-2,6-diyl,bicyclo 2.2.2!octane-1,4-diyl or 1,3-dioxoborinane-2,5-diyl; M¹, M² andM³ are identical or different and are each --CH₂ --CH₂ --, --CH═CH--,--C.tbd.C--, --CH₂ CH₂ CH₂ CH₂ --, --CH₂ CH₂ CH₂ --O--, --O--CH₂ CH₂ CH₂--, --CH₂ CH₂ --CO--O--, --O--CO--CH₂ CH₂ --, --CH₂ --O--, --O--CH₂ --,--CO--O-- or --O--CO--; k, l, m, n, o and p are zero or one, with theproviso that the sum k+m+o is greater than zero; X¹, X², X³ and X⁴ are,independently of one another, H, F or Cl; and Y is --CH₂ F or --CHF₂. 2.A compound according to claim 1 wherein Y is --CHF₂.
 3. A compoundaccording to claim 1, wherein X² and X⁴ are both F.
 4. A compoundaccording to claim 1, wherein:R¹ is a straight-chain alkyl having 1 to15 carbon atoms; A¹, A² and A³ are identical or different and are1,4-phenylene, in which one or two H atoms are optionally replaced by F,or trans-1,4-cyclohexylene; and X¹, X², X³ and X⁴ are, independently ofone another, H or F.
 5. A compound according to claim 4, wherein Y is--CHF₂.
 6. A process for the preparation of a compound of the formula(I) in claim 1, which comprises:a) reacting a compound of the formula(II) ##STR77## where Z¹ =R¹ (--A¹)_(k) (--M¹)_(l) (--A²)_(m) (--M²)_(n)(--A³)_(o) (--M³)_(p), andW=Br, I with trimethylsilylacetylene in across-coupling reaction catalyzed by a palladium compound; b)desilylating the compound from a) with a base; c) deprotonating thecompound from b); d) reacting the compound from c) withdimethylformamide or formaldehyde, and e) fluorinating the resultantaldehyde or alcohol by reaction with diethylaminosulfur trifluoride. 7.A liquid-crystal mixture comprising one or more compounds of the formula(I) of claim 1 as liquid crystal component(s).
 8. A liquid-crystalmixture according to claim 7 which exhibits nematic properties.
 9. Aliquid-crystal mixture according to claim 7, which comprises 1 to 8compounds of the formula (I).
 10. A liquid-crystal mixture according toclaim 7, which comprises 0.1 to 70 mol % of one or more compounds of theformula (I).
 11. An electro-optical switching or display device whichcomprises a liquid-crystal layer of a composition of claim
 7. 12. Thedevice of claim 11, which further comprises at least one alignmentlayer, at least one polarizer, electrodes and outer plates.
 13. Thedevice of claim 11, which is a super-twisted nematic display or thinfilm transistor-twisted nematic display.
 14. A fluoroalkylethynyl- ordifluoroalkylethynyl- benzene compound of one of the following formulae:##STR78## in which R¹ is H, a straight-chain or branched (with orwithout an asymmetric carbon atom) alkyl group having 1 to 15 carbonatoms, in which one or two non-adjacent CH₂ groups are optionallyreplaced by --O--, --S--, --CO--, --CO--O--, --O--CO--, --CO--S--,--S--CO--, --O--CO--O--, --CH═CH--, --C.tbd.C--, cyclopropane-1,2-diylor --Si(CH₃)₂ --, and in which one or more H atoms in the alkyl groupare optionally replaced by F, Cl or CN;Y is --CH₂ F or --CHF₂, and L¹and L² are each, independently of one another, H or F.
 15. A compoundaccording to claim 14, wherein Y is --CHF₂.
 16. A compound according toclaim 14, wherein L¹ and L² are both F.
 17. A liquid-crystal mixturecomprising one or more compounds of claim 14 as liquid crystalcomponent(s).
 18. A liquid-crystal mixture according to claim 17 whichexhibits nematic properties.
 19. An electro-optical switching or displaydevice which comprises a liquid-crystal layer of a composition of claim17.
 20. The device of claim 19, which further comprises at least onealignment layer, at least one polarizer, electrodes and outer plates.21. A fluoroalkylethynyl- or difluoroalkylethynyl- benzene compound ofthe formula (I) ##STR79## wherein: R¹ is H, a straight-chain or branched(with or without an asymmetric carbon atom) alkyl group having 1 to 15carbon atoms, in which one or two non-adjacent CH₂ groups are optionallyreplaced by --O--, --S--, --CO--, --CO--O--, --O--CO--, --CO--S--,--S--CO--, --O--CO--O--, --CH═CH--, --C.tbd.C--, cyclopropane-1,2-diylor --Si(CH₃)₂ --, and in which one or more H atoms in the alkyl groupare optionally replaced by F, Cl or CN;A¹, A² and A³ are identical ordifferent and are 1,4-phenylene, pyrazine-2,5-diyl, pyridine-2,5-diyl orpyrimidine-2,5-diyl, in which one or two H atoms are optionally replacedby F, or trans-1,4-cyclohexylene, 1,3,4-thiadiazole-2,5-diyl,1,3-dioxane-2,5-diyl, naphthalene-2,6-diyl, bicyclo2.2.2!octane-1,4-diyl or 1,3-dioxoborinane-2,5-diyl; M¹, M² and M³ areidentical or different and are each --CH₂ --CH₂ --, --CH═CH--,--C.tbd.--C--, --CH₂ CH₂ CH₂ CH₂ --, --CH₂ CH₂ CH₂ --O--, --O--CH₂ CH₂CH₂ --, --CH₂ CH₂ --CO--O--, --O--CO--CH₂ CH₂ --, --CH₂ --O--, --O--CH₂--, --CO--O-- or --O--CO--; k, l, m, n, o and p are zero or one, withthe proviso that the sum k+m+o is greater than zero; X¹ and X³ are,independently of one another, H; X² and X⁴ are independently of oneanother, F; and Y is --CHF₂.